1. Field of the Invention
This invention relates to an electric rotary machine having a concentrated winding stator and, more particularly, relates to an electric rotary machine having a concentrated winding stator in which the cogging torque is reduced remarkably, the motor torque is high, the vibration and noise are small and the efficiency is high.
2. Description of the Prior Art
The cogging torque is in inverse proportion to the least common multiple of a pole number P of a permanent magnet rotor and a magnetic pole number M of a stator. The Japanese Patent Laid-Open No. 247/1992 discloses an electric motor having three pieces of a stator magnetic pole in a group A and three pieces of a stator magnetic pole in a group B, and six pieces of a stator magnetic pole in a group C. In the electric motor, the pole number P of the permanent magnet rotor is 16, and the pole number M of the stator is 12. Each pole piece of the stator magnetic pole in the group A has at both sides thereof, respectively, the same small projections extending in the circumferential direction of the rotor. Each pole piece of the stator magnetic pole in the group B has at the left side thereof a small projection extending in the circumferential direction of the rotor and at the right side thereof a large projection extending in the circumferential direction of the rotor. Each pole piece of the stator magnetic pole in the group C has at the right side thereof a small projections extending in the circumferential direction of the rotor and at the left side thereof a large projection extending in the circumferential direction of the rotor.
Further, the Japanese Patent Laid-Open No. 110468/1987 discloses a brushless motor in which P:M=6n.+-.2:6n, where n is an integer not less than 2, P is the pole number of the permanent magnet rotor and M is the pole number of the stator.
However, in the electric motor shown in the Japanese Patent Laid-Open No. 247/1992, the winding coefficient indicating the effective application of the winding is small, such as 0.866 and the motor torque is small, because the pole number P of the permanent magnet rotor is 16, and the pole number M of the stator is 12. Further, the least common multiple of the pole number P of said permanent magnet rotor and the pole number M of said stator relating to the cogging torque is small, such as 48, so that the cogging torque cannot be reduced remarkably. That is, in case that the magnitude of the cogging torque of the motor which is similar in construction to the conventional motor is 1.0, and the figure of the pole piece is specified, the reduction effect of the cogging torque (a ratio of amplitude of the cogging torque) is as small as 0.35, or the peak to peak value of the cogging torque shown in FIG. 11A is only reduced by 1/3 as shown in FIG. 11B.
Further, the fundamental frequency f of the motor at a predetermined revolution is high (for example, f=(1800/60).times.8=240 Hz at 1800 rpm) and the iron loss generated in the stator magnetic pole is large (it is proportion to f.sup.1.5.about.f.sup.1.7) because the pole number P of the permanent magnet rotor is large, such as 16, so that the efficiency of the motor is small.
Further, the motor torque is small, because the length of at least one projection of the pole piece is small in the group A, B or C, so that the quantity of the magnetic flux interlinking to the winding becomes small. Furthermore, there is such a defect that the winding inserted into the slot comes out so that the turn number of the winding, the occupation rate of the winding and the efficiency of the motor become small, because the length of at least one projection of the pole piece is small so that the slit width formed between the projections facing each other becomes large.
In the motor shown in the Japanese Patent Laid-Open No. 110468/1987, P:M=6n.+-.2:6n, for example, P:M=14:12 or 10:12, so that the winding factor becomes large, such as 0.933 and the large motor torque is generated, where n is an integer not less than 2, P is the pole number of the permanent magnet rotor and M is the pole number of the stator.
Further, the least common multiple relating to the cogging torque is 84 in case that P:M=14:12, and is 60 in case that P:M=10:12. The cogging torque which is inverse proportion to the least common multiple is smaller than that in the motor shown in the Japanese Patent Laid-Open No. 247/1992. However, in case of the motor shown in the Japanese Patent Laid-Open No. 110468/1987 wherein P:M=14:12, the least common multiple is 84 so that the reduction effect (a ratio of amplitude of the cogging torque) is 0.57 (=1.0.times.48/84). In case that P:M=10:12, the least common multiple is 60 so that the reduction effect is 0.8 (=1.0.times.48/60). That is, the motor shown in the Japanese Patent Laid-Open No. 110468/1987 is lower in reduction effect and larger in fluctuation of the revolution, vibration and noise than that of the motor shown in the Japanese Patent Laid-Open No. 247/1992.